If you want to achieve this in the PI Trend control, then it is won't happen unless you are trending an AF Attribute but even then it is a bit "messy".

If you were to create a .Net add-in that could add tooltip text (or an information box) when a cursor is dropped on a trend, then you take the trace configuration (if a PI Point then look at the "EngUnits" point attribute, AF Attribute look at the UOM) and convert to a different UOM and display it - if you have AF then you don't need conversion algorithms in your add-in, just use the UOM class of AFSDK.

If you were to create a .Net add-in that could add tooltip text (or an information box) when a cursor is dropped on a trend

Rhys,

I've checked ProcessBook VBA Reference and haven't found any possibility to modify text when cursor is dropping. It's only avant called ToolTipText - it's can be used for tool tip modify, but there's not the case...

There is a problem because I have MDB and AF is not implemented in our PI System.

As Rhys pointed out, you will need to implement your own parsing and conversion algorithms if you do not want to use AF. The "engunits" PI Point attribute is a string attribute and can be set to arbitrary values by system administrator; as an example, "degrees Celcius" could be represented as "C", "DEG.C", "Degrees C", etcsoft. Same holds true for other units of measure, let alone the language aspect and whether these are SI units or not.

This exact issue is a key one addressed by the PI Asset Framework (AF), which helps organize, provide context, find, standardize, etc. all the information collected in the PI Data Archive, which stores and serves masses of data. We had a good discussion about this in this discussion thread.

On the "MDB or AF" topic, note that AF is now included with PI Server 2010. The migration from PI Module Database (MDB) to AF is taken care of by the upgrade process. I would like to invite you to watch the recording of the "Your Personal PI System 2010" webinar to hear more about the transition from MDB to AF from a system management standpoint, and the "Transitioning from ModuleDB to AF" webinar to get more of the programmer's point of view.